1. Overview
A three-phase AC induction motor (SIMO) is a device that converts electrical energy into mechanical energy based on the principle of electromagnetic induction. Its stator windings are fed three-phase AC power with a phase shift of 120°, generating a rotating magnetic field that drives the rotor conductors to induce current and generate torque. This motor features a robust structure, reliable operation, and easy maintenance, making it the most widely used power source in industry.

2. Core Structure and Operating Principle

Stator:
The core is composed of laminated high-permeability silicon steel sheets. Three sets of windings (U, V, and W) are spatially symmetrically distributed (with a phase shift of 120°).
When three-phase AC power is applied to the windings, a composite magnetic field with constant amplitude and continuously rotating direction is generated (synchronous speed n_s = 120f / p, where f is the power frequency and p is the number of magnetic pole pairs).

Rotor:
Squirrel Cage: Uninsulated conductor bars are embedded in the core slots, connected at both ends by short-circuit rings. Simple and robust structure, low cost, and dominant in industrial applications.
Wound Rotor: Three-phase insulated windings are embedded in the core slots, connected to external variable resistors via slip rings and brushes. They offer high starting torque and good speed regulation, making them suitable for specific applications.
The rotating magnetic field cuts through the rotor bars, inducing electromotive force and current. The current-carrying conductors are subjected to forces (Lorentz forces) in the magnetic field, generating electromagnetic torque that drives the rotor. The rotor speed n is always lower than the synchronous speed n_s (due to slip s = (n_s - n) / n_s).

Casing and end caps: Provide mechanical support, protect internal structures, and dissipate heat. Common protection levels (IP codes) meet different environmental requirements.

Bearings: Support the rotor shaft and reduce friction. Regular maintenance and lubrication are required.

Cooling System: Self-cooling (IC 411) is commonly used, while some high-power or special environments use forced air or water cooling (IC 416/IC 666, etc.).

Terminal Box: Contains terminals for connecting power cables (wye or delta).

3. Key Performance Parameters

Rated Power: The continuous mechanical power output at the motor shaft (in kW or HP), typically ranging from a few kilowatts to several megawatts.

Rated Voltage: The designed operating voltage (e.g., 380V, 415V, 480V, 690V), which must match the power supply system.

Rated Frequency: The designed operating frequency (50Hz or 60Hz).

Rated Speed: The rotor speed (rpm) at rated power output, determined by the number of poles and slip (e.g., approximately 2880-2970 rpm @ 50Hz for a 2-pole motor).

Rated Current: The line current in the stator winding (A) at rated power output.

Efficiency: The percentage of mechanical output power to electrical input power. International standards (such as IEC 60034-30) define efficiency classes (IE1, IE2, IE3, and IE4), with IE4 being the most efficient.

Power Factor: The ratio of input active power to apparent power, reflecting reactive power demand. Typically ranges from 0.8 to 0.9 (at full load).

Starting Current: The peak current at the moment a motor starts (typically 5 to 7 times the rated current).

Starting Torque: The torque generated by a motor during startup (typically 1.5 to 2.5 times the rated torque).

Breakdown Torque: The maximum torque a motor can produce without stalling (typically 2 to 3 times the rated torque).

Torque-Speed Characteristics: A curve describing the motor's ability to output torque at different speeds.

Protection Rating (IP Rating): Defined by IEC 60529, this rating indicates the enclosure's ability to protect against solid foreign objects and water intrusion (e.g., IP55, IP56).

Insulation Class: Defined by IEC 60085, this rating indicates the thermal resistance of the winding insulation material (e.g., Class B, F, H), which determines the allowable temperature rise.

4. Typical Applications

Industrial Manufacturing: Drives for pumps, fans, compressors, conveyor belts, machine tools, crushers, mixers, extruders, etc.

Infrastructure: Heating, ventilation, and air conditioning (HVAC) system fans/pumps, water treatment plant pump stations, and elevator traction machines.

Energy and Power: Power plant auxiliary equipment (feedwater pumps, induced draft fans), and pumps and compressors in the oil and gas industry.

Transportation: Port cranes and auxiliary systems (non-main drive) for electric vehicles.

Other: Agricultural irrigation pumps, mining machinery, etc.

5. Selection and Usage Considerations

Load Matching: The power, speed, and torque characteristics must meet the load requirements. Avoid prolonged severe overloading or underloading.

Voltage and frequency: Must match the power supply. Voltage tolerance is typically ±5%, and frequency tolerance is ±2%.

Environmental conditions: Consider ambient temperature, altitude (which affects cooling), humidity, dust, corrosive gases, and explosion-hazardous areas (explosion-proof certification required), and select the appropriate protection level, housing material, and cooling method.

Starting method: Based on the grid capacity and starting current requirements, select direct-on-line starting, star-delta starting, soft starter, or inverter.

Mounting method: Based on standards (IEC 60034-7, NEMA MG1), select B3 (horizontal foot mount), B5 (flange mount), or B35 (foot + flange).

Maintenance requirements: Consider accessibility for routine maintenance, such as bearing lubrication cycles, cooling duct cleaning, and wiring tightness inspection.

6. Maintenance Basics

Regular Inspection: Clean the motor surface and cooling ducts (especially for self-ventilated motors); inspect fasteners (anchor bolts, terminal blocks); and monitor for abnormal noise/vibration.

Bearing Maintenance: Re-lubricate or replace grease according to the manufacturer's manual's specified intervals and grease brand. Excessive grease may cause overheating.

Insulation Resistance Test: Measure the winding-to-ground and phase-to-phase insulation resistance using a megohmmeter regularly (e.g., annually) to ensure compliance with safety standards.

Operation Monitoring: Monitor operating current (to avoid overload), temperature rise (measure the housing temperature, refer to the insulation class's allowable value), and vibration.

7. Safety Standards

Installation, operation, and maintenance must comply with the electrical safety standards of the country/region (e.g., IEC, NEC, GB standards).

Ensure the motor is reliably grounded (PE conductor).

Disconnect the power supply and perform an electrical test before performing any internal maintenance work.

Use certified explosion-proof motors (e.g., those complying with ATEX or IECEx standards) in flammable and explosive environments.

Three-phase AC induction motors, with their ruggedness, reliability, and standardized design, continue to provide a core driving force for global industry. Understanding their structural principles, performance parameters, and proper selection and maintenance methods are crucial to ensuring long-term, stable operation. In practical applications, strictly adhere to manufacturer specifications and safety standards.